feat(stdlib/E6a): per-decl nominal identity for enum + union decls

Give top-level ENUM and UNION decls per-decl nominal identity so two
same-name flat enums/unions intern DISTINCT nominal TypeIds instead of
collapsing to one global last-wins entry. Establishes the reusable
non-struct register path the later E6 kind-steps (E6b error-set, E6c
protocol, E6d foreign-class) extend.

Registration side (was: stateless `type_bridge.resolveInlineEnum/Union`
`findByName` last-wins short-circuit, no Lowering access):
- Split the type_bridge inline builders into a body-BUILDER
  (`buildEnumInfo` / `buildUnionInfo`) + the existing thin interner
  wrappers (field-type positions keep the legacy single-slot path).
- Add `Lowering.registerEnumDecl` / `registerUnionDecl` mirroring
  `registerStructDecl`: build the TypeInfo, intern via
  `internNamedTypeDecl(decl_key, name_id, info, nominal_id)` under the
  per-decl nominal identity (reserved slot id, else `shadowNominalId`).
- Reroute all six enum/union registration dispatch sites (scanDecls
  const-wrapped + top-level, lowerDecls/comptime, block-local, local
  const) to the new path.

Shared infra generalized ONCE:
- Pass-0b genuine-shadow pre-pass now reserves struct/enum/union shadow
  slots of the MATCHING kind, grouped by (kind, name), via a kind-generic
  `topLevelTypeDecl` / `reserveShadowSlot`. A forward/self/mutual ref to a
  shadow name binds to the reserved nominal TypeId.
- `namedRefTid` consults `type_decl_tids` for `.enum_decl`/`.union_decl`
  before the global `findByName`.

No new per-kind resolution path: selectNominalLeaf / headTypeGate /
flatTypeAuthorCount already gate every kind. Single-author /
phantom-double-spelling names keep nominal_id 0 (byte-identical corpus).

Regressions 0795-0798 (enum + union: ambiguity over every bare-type form,
and own-wins with distinct nominal TypeIds), fail-before/pass-after:
0795/0797 exit 0 -> exit 1 with the loud "type is ambiguous" diagnostic;
0796 silently printed `own=.east` -> correct `own=.north`; 0798 hard
`field 'm' not found` error -> correct `own=5 dep=9`.

Gate: zig build && zig build test (423/423) && run_examples.sh (537/537)
all exit 0; m3te ios-sim build via the main binary exit 0.

examples/expected/0795-modules-same-name-enum-ambiguous.exit

@@ -0,0 +1 @@
+1

examples/expected/0795-modules-same-name-enum-ambiguous.stderr

@@ -0,0 +1,23 @@
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:32:19
+   |
+32 |     sz := size_of(Dir);
+   |                   ^^^
+
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:33:9
+   |
+33 |     d : Dir = .north;
+   |         ^^^
+
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:34:16
+   |
+34 |     t : Type = Dir;
+   |                ^^^
+
+error: type 'Dir' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0795-modules-same-name-enum-ambiguous.sx:25:14
+   |
+25 |         case Dir: 1;
+   |              ^^^

examples/expected/0795-modules-same-name-enum-ambiguous.stdout

@@ -0,0 +1 @@
+

examples/expected/0796-modules-same-name-enum-own-wins.exit

@@ -0,0 +1 @@
+0

examples/expected/0796-modules-same-name-enum-own-wins.stderr

@@ -0,0 +1 @@
+

examples/expected/0796-modules-same-name-enum-own-wins.stdout

@@ -0,0 +1 @@
+own=.north dep=.west

examples/expected/0797-modules-same-name-union-ambiguous.exit

@@ -0,0 +1 @@
+1

examples/expected/0797-modules-same-name-union-ambiguous.stderr

@@ -0,0 +1,23 @@
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:31:19
+   |
+31 |     sz := size_of(Pair);
+   |                   ^^^^
+
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:32:9
+   |
+32 |     u : Pair = ---;
+   |         ^^^^
+
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:33:16
+   |
+33 |     t : Type = Pair;
+   |                ^^^^
+
+error: type 'Pair' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0797-modules-same-name-union-ambiguous.sx:24:14
+   |
+24 |         case Pair: 1;
+   |              ^^^^

examples/expected/0797-modules-same-name-union-ambiguous.stdout

@@ -0,0 +1 @@
+

examples/expected/0798-modules-same-name-union-own-wins.exit

@@ -0,0 +1 @@
+0

examples/expected/0798-modules-same-name-union-own-wins.stderr

@@ -0,0 +1 @@
+

examples/expected/0798-modules-same-name-union-own-wins.stdout

@@ -0,0 +1 @@
+own=5 dep=9